Under applicable safety codes, the National Fire Codes Standards for Ovens and Furnaces NFPA, industrial ovens and dryers may operate with a concentration of flammable components of up to twenty-five percent (25%) of the lower flammable limit (LFL). In fact, an exception within the code allows operation at up to fifty percent (50%) of the LFL where a continuous vapor concentration indicator and a controller are provided.
In practice, however, most industrial ovens and dryers typically operate with concentrations of flammable components at approximately ten percent (10%) of the LFL. The fact that the typical industrial level is much lower than allowable is caused by practical situations where the covering area heated, the thickness of the coating being dried, the speed of the operation or the width of the process web are always operating at less than design. Since such low levels indicate greater fire safety in the operation, there has been little or no impetus to increase the concentration levels.
Within recent years, however, laws have been enacted to prohibit pollution, and to control the emission of volatile organic substances. Typically, state and federal laws and regulations require that the release of organic substances into the atmosphere be reduced or even totally stopped. Solvent drying processes, therefore, require that their exhaust be controlled by a solvent absorption system or an incinerator so that all organic solvent vapors are either collected for reuse or are destroyed by incineration.
Addition of an incinerator or solvent recovery system to a solvent drying process involves initial capital costs and higher operating costs. These costs are directly related to the quantity of gas handled by the abatement system. In order to make the design of an abatement system as economical as possible, one principal objective in the design of the system is to reduce the amount of air handled by the system. A system design, therefore, that allows an increase in the solvent concentration from ten percent (10%) to thirty percent (30%) of the LFL reduces the airflow to one-third of the previous flow rate. The capital cost for an abatement system would be reduced to less than one-half, and the operating costs would be reduced to about one-fifth for such a system.
Operating within the economic requirements of an abatement system, therefore, mandates operation at higher concentrations of solvent vapors. Such operation, however, reduces the margin of safety and requires a higher reliability and accuracy from a solvent vapor analyzer to conform to the law.
To insure accuracy and reliability, a solvent vapor analyzer should meet the following requirements:
1. The response time should be very fast, and the analysis of a sample should be available in two seconds or less. The response may be longer, if the particular industrial application monitored will not approach the flammable limit faster than the solvent vapor analyzer can control or shut down the process.
2. The analyzer indicator and alarms should be accurate over wide changes in solvent composition.
3. None of the vapor in the sample should be allowed to condense. All vapors follow the scientific law of partial pressure and have a specific vapor concentration per temperature curve. When the vapor is sampled from a particular industrial application at some elevated temperature, the vapor sample should be heated so that the sample does not approach its dew point within the analyzer. Operation of a solvent vapor analyzer above the vapor flash point is usually sufficient. When other constituents, like plasticizers, oils and mists are present, analyzer temperatures close to or at the dryer operating temperature are typically required.
4. Such high temperatures do not protect the sample system from the fouling by polymer products that varnish and gunk the flow components. The flow components, therefore, should then be easily removable for cleaning and all of the passages should be accessible and as large and easy to clean as feasible.
5. The vapor analyzer system and all of its component parts must be limited in number and simple in design in order to provide the highest reliability.
6. The design should meet all code requirements as found in the National Fire Protection Association, NFPA 86 Standard for Ovens and Furnaces, and NFPA 70 National Electrical Code; and American National Standard ANSI/ISA-S.12.13, Performance Requirements, Combustible Gas Detectors.
7. The design should be reliable and operate with a fail-safe design.
Solvent vapor analyzers in industry today use a variety of detectors. The flame ionization detector uses a hydrogen flame where the sample is injected into the burner and the carbon atoms present in the flame produce a measurable ion current utilizing a high voltage and an electrometer. The signal present in the detector is related to the concentration of carbon atoms. Instruments using the flame ionization principle are the Ratfisch and Bernath Atomic. These two instruments are similar in the detector design. The Bernath Atomic mounts at the sample location and the Ratfisch uses a long, heated sample line.
These flame ionization detectors were designed to perform hydrocarbon analysis and were intended to measure parts per million of hydrocarbons, but they have been used in monitoring flammable vapors. Flame ionization detectors have two serious drawbacks. Most important is the fact that they require recalibration whenever the solvent mixture is changed. The calibration is then correlated to percentage of flammable limit from parts per million hydrocarbon or carbon. This correlation is affected by the carbon molecular bonds and the presence of oxygen and other elements in the molecule causing a calibration shift with solvent formulation changes. Secondly, these flame ionization detectors employ pumps to inject a sample through capillaries into a hydrogen flame. This kind of flow system is susceptible to plugging and maintenance problems, particularly in industrial applications such as solvent drying and the like.
Sensors using the principle of catalytic combustion are frequently used in area monitoring. Their use in process monitoring is limited by their susceptibility to poisoning and deterioration by "catalytic poison,"--a group of substances that either coat or corrode the bead catalytic element. Such substances are frequently found in drying operations. To guarantee that the catalytic sensor is working properly, frequent calibration tests must be run. Even so, under certain circumstances a sensor can fail within hours and the only way to know if it has failed is by testing it with a known concentration of gas. Catalytic sensors do have a better cross calibration accuracy than the flame ionization detectors. This deficiency is addressed in U.S. Pat. Nos. 4,116,612 and 4,322,964.
Infrared spectrophotometer systems also require calibration when solvent mixtures are changed. They are not well suited for industrial applications such as oven monitoring because maintenance of the optics, which require frequent alignment and cleaning, is excessive in such applications and high temperature operation is limited.
A flame temperature analyzer, such as those manufactured by Control Instruments, Inc. uses the principle of incineration. A small pilot flame inside an explosion proof cell is continuously exposed to the sample. When the sample is air, a temperature detector indicates the heat released from the small pilot flame. When the sample contains flammable vapors, the temperature detector produces a signal in proportion to 0-50% of the lower flammable limit. The characteristics of the sensing flame detector are such that the signal produced is proportional for the lower flammable limits of a wide range of solvent vapors. The flame temperature analyzer, then, is accurate for measuring the percentage of the lower flammable limit even when the solvent vapor composition is changed.
A typical prior art flame temperature analyzer meets all of the requirements for accuracy, but has been limited in its response time because its remote location has required the use of heated sample lines which create a sample response delay. Further, the sample filter of the prior art flame temperature analyzer must filter the whole of the sample flow. Such filters, therefore, require large volumes, slowing sample delivery and are subject to frequent replacement in applications where dust or gunking is prevalent. The use of individual parts for each function typically creates a piping jungle and makes maintenance difficult thereby reducing reliability. The detector principle itself, however, is simple and reliable. It is, therefore, an object of the present invention to provide the improvements needed to enable a device with the accuracy and reliability of the flame temperature analyzer to be employed for the safe operation cf processes using flammable vapors.
The prior art appears to disclose no unitary apparatus for scientifically determining with accuracy and reliability the LFL in processes that use high flash point solvents or contain low vapor pressure sample components that foul an analyzer. Such solvents and sample components are found in industrial applications for the making of vinyl tile and sheet goods, masking tape and web offset printing.
U.S. Pat. No. 4,336,721 entitled "GAS ANALYZER" discloses a heated analyzer, mountable on a duct, which uses an aspirator for sucking in the sample. This gas sampling apparatus uses an inlet tube, a nozzle through which the gas sample is metered. The sample is then drawn past the cell by the aspirator. The apparatus is heated above 430.degree. C. to avoid the accumulation of deposits and below 704.degree. C. to prevent the slagging of molten ash on the surfaces of the cell. The flow system is a simple once-through arrangement. The detector output is not subject to change when the flow changes, since it operates on the partial pressure of oxygen. The apparatus works well with its self cleaning objectives but does not appear to be appropriate for solvent vapors because it operates at temperatures that would decompose solvent vapors which should cause a loss of reading and accuracy.
U.S. Pat. No. 4,336,722 entitled "METHOD AND APPARATUS FOR SAMPLING WASTE GASES" describes a similar device in which an aspirator is used to draw the sample from a source. The exhaust of the aspirator is discharged through a concentric tube in the center of the sample probe. The object of this probe and apparatus is to cool and condense the sample so that vapor is removed and only gases are delivered to the gas monitor. The adiabatic cooling chamber and the discharge tube inside the sample probe are means for obtaining the greatest sample cooling which is inappropriate for the analyzer specifications.
Additional patents using aspiration or heated probes are inappropriate for the present application. U.S. Pat. No. 4,379,412 is a high temperature probe with blow-back means for clearing the probe and filter. U.S. Pat. No. 3,106,843 is a steam aspirated sample probe requiring downstream condensation for operation, similar also to U.S. Pat. No. 2,987,921 where water washes the sample probe then a steam aspirator draws and cleans the sample. U.S. Pat. No. 3,593,023 also uses a condenser and aspirator. All these devices are believed to be inappropriate for the needs of the present invention.
U.S. Pat. No. 4,128,458 entitled "COMBUSTILE ELEMENT AND OXYGEN CONCENTRATION SENSOR" utilizes divided flow to operate an oxygen cell and a catalytic combustible cell. The flow was divided so that air may be added to the combustible sample to supply the oxygen required for the operation of the catalytic oxidation of the sample. A third conduit serves to carry extra sample gas to the aspirator as a means to load the aspirator and reduce the flow variations through the two detector cells. This third conduit produces no regulation because its shunting action proportionally supplies a volume of sample to meet the demand requirements of the aspirator. A similar effect could be obtained by reducing the size of the aspirator.
U.S. Pat. No. 4,317,379 entitled "PROCESS AND APPARATUS FOR THE CONTINUOUS WITHDRAWAL OF SPECIMENS FROM A CURRENT OF A CRUDE GAS FOR THE PURPOSE OF GAS ANALYSIS" preconditions the crude sample by cooling. The condensate is heated and ejected back into the sample source using a steam ejector. A branch of the high pressure sample is filtered, then pressure reduced for the analyzer. This is a reconditioning and sample cleaning apparatus whose function would destroy the accuracy of a solvent vapor analyzer.
U.S. Pat. No. 4,342,234 entitled "APPARATUS FOR EXTRACTING HOT GAS SAMPLE FROM A CHAMBER AND FOR FEEDING THE SAMPLE TO AN ANALYZER" utilizes a chamber and a sample probe surrounded by a heating jacket leading directly to a heated analyzer. This analyzer, known commercially as the Bernath Atomic utilizes a mechanical pump, capillaries for sample injection and sample bypass and a flame ionization detector. The flame ionization detector is seriously affected by changes in solvent composition. The capillaries are subject to fouling and the mechanical pump is complex, therefore the requirements for accuracy and reliability are not believed to be met.
U.S. Pat. No. 4,116,612 entitled "GAS MONITOR SYSTEM" is a self-check system to frequently introduce test gas into the gas monitoring system to determine whether the analyzer readings are in or out of calibration.
U.S. Pat. No. 4,322,964 entitled "GAS ANALYZER CALIBRATION APPARATUS" is a fluid gate on the sample probe that enables the introduction of calibration gas as outlined in U.S. Pat. No. 4,116,612. The purpose of this apparatus is to prove that the solvent vapor analyzer is accurate; if not the process is shut down and the analyzer malfunction alarmed. The need for apparatus of this design is deleted when the sample system and the detector perform accurately and reliably as found in our novel apparatus.